I think most mobile phones have a timer, I have an all singing & dancing model it does everything basic (It's the MOST basic model available).

Never had to do a 30 minute exposure, longest so far has been 10 minutes, with no watch (don't own one), no phone, I just walked around talking to myself counting of the minutes, worked perfectly.

A watch is accurate enough, 30 seconds either way is irrelevant.
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Ian